Update on Plastid Biotechnology Plastid Biotechnology: Food, Fuel, and Medicine for the 21st Century

DNA-containing cellular compartments in plant cells are the nucleus, plastids, and mitochondria. The nuclear genome, encoding approximately 29,000 to 32,000 genes, is the most common target for biotechnological applications. The number of genes encoded by the plastid and mitochondrial genomes is much smaller, approximately 120 and approximately 60, respectively. The number of processes that may be modified by engineering the organellar genomes is much higher than the number of genes would suggest. That is because approximately 10% of the nuclear gene products are targeted to plastids and approximately 10% to mitochondria (Leister, 2003), making these nucleus-encoded processes amenable to manipulation by plastome engineering. Traits that may be engineered by plastid genome manipulation are not restricted by the plant’s gene content, because incorporating new genes or gene clusters from heterologous sources may expand the plastid’s biosynthetic repertoire beyond what is provided by nature. Engineering of the plastid genomes (plastomes; plastid DNA or ptDNA) was first accomplished in Chlamydomonas reinhardtii, a unicellular green alga (Boynton et al., 1988), followed by plastid transformation in tobacco (Nicotiana tabacum), a flowering plant species (Svab et al., 1990). Since 1988, plastid transformation has been expanded to a diverse group of species (see below). However, commercial applications are lagging behind, and currently no crops are grown commercially utilizing this technology. This review focuses on the principles of plastome engineering and highlights recent developments. Most examples will be described in tobacco, which is the model species of plastid engineering. Additional information on the biotechnological applications of plastid transformation can be found in recent reviews (Bock, 2007; Daniell et al., 2009; Cardi et al., 2010).